Compared with traditional liquid crystal panels, the Active Matrix/Organic Light Emitting Diode (AMOLED) display panel has characteristics such as faster response, larger contrast, wider angle of view, and so on, and thus has drawn more and more attention to display technology developers.
The AMOLED display panel can emit light due to being driven by current generated by a driving Thin Film Transistor (TFT) in the saturation state. Different currents would be generated due to different threshold voltages when the same gray-scale voltage is input, which results in nonuniformity of driving current. The Low Temperature Poly-Silicon (LTPS) has very poor uniformity of threshold voltage Vth in the manufacturing procedure, and the threshold voltage Vth also drifts; therefore, conventional 2T1C circuits have very poor uniformity of brightness.
FIG. 1 illustrates a voltage driven pixel driving circuit structure (2T1C) composed of 2 TFT transistors and 1 capacitors as known. The first transistor M1 transports the voltage on the data line to the gate of the driving transistor DTFT which transforms this data voltage into corresponding current to be supplied to the organic light emitting diode. During normal operation, the driving transistor DTFT should operate at the saturation region to supply stable driving current within the scanning period of one row. The driving current (i.e., the current flowing through the OLED) can be expressed as:
      I    OLED    =            1      2        ⁢                  μ        n            ·              C        OX            ·              W        L            ·                        (                                    V              data                        -                          V              oled                        -                          V              th                                )                2            wherein μn is the carrier mobility, Cox is the capacitance of the gate oxide layer, W/L is the aspect ratio of the transistor, Vdata is the data voltage, Voled is the operating voltage of the OLED which is shared by all pixel units, and Vth is the threshold voltage of the driving transistor DTFT. As can be seen from the above expression, if Vth is different between different pixel units, it results in that the driving current would be different; if the threshold voltage Vth of the driving transistor drifts with time, it may results that current at former time would be different from that at latter time, which causes afterimage, reduces display effect, and influences users' experience.